1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device such as a liquid crystal projector and more particularly to improvements in light-shielding capability of an active matrix type liquid crystal display (LCD) device for a light valve wherein switching of a liquid crystal is carried out through thin film transistors (TFTs).
2. Description of the Related Art
In recent years, as a display for the wall-hanging type television, the projective-type television or the OA appliances, various display units using a liquid crystal panel have been being developed. Among those liquid crystal panels, an active matrix type LCD wherein TFTs are included as the active element in a LCD device is the most promising to realize a high quality display unit for the OA appliances as well as a display unit for the high definition television because of its advantageous natures such as the one that an increase in number of scanning lines therein does not result in a deterioration of the contrast or the response time thereof. Especially when applied to a projection type LCD with the liquid crystal projection or the like, it allows achieving a large screen display with ease.
Normally, in the active matrix type LCD device for a light valve that is utilized for the liquid crystal projection, a small element is illuminated with a strong light, and the light passing therethrough is controlled according to the image data by turning on and off each pixel separately through switching of a liquid crystal by a corresponding TFT, and then the transmitted light, being magnified by an optical element such as a lens, is projected on a screen or the like. At this, if an active layer of the TFT is formed from polysilicon (p-Si), there arises a problem of the leakage current in a channel section of the TFT at the off-time that may be produced, due to the photo-excitation, by the reflected light from the optical system such as a lens, let alone under the influence of the incident light thereon.
The conventional active matrix type LCD device for a light valve of this sort has a first light-shielding film set on the TFT substrate and a second light-shielding film set above the TFT. Upon this, when the incident light comes from the side of an opposite substrate facing the TFT over a liquid crystal layer, the second light-shielding film cuts off the incident light and the first light-shielding film, the reflected light from the optical system.
The second light-shielding film is formed, in some cases, on the same substrate as the TFT with an interlayer film inserted therebetween and, in other cases, on the opposite substrate to the TFT with the liquid crystal layer inserted therebetween. In the case that the second light-shielding film is formed on the opposite substrate to the TFT, the second light-shielding film must be made larger than the first light-shielding film to allow 10 xcexcm or so for a shift in the overlay accuracy between two substrates. This brings about a problem that the aperture ratio cannot be made sufficiently large.
As a result, the method in which the second light-shielding film is formed on the identical substrate with the TFT is mainly employed at present. In this instance, it is unnecessary to allow such a large margin as described above since a high alignment accuracy can be obtained by making a good use of a manufacturing process of a semiconductor device. However, this method pays no regard to the positioning relation among these two light-shielding films and the TFT so that measures of shielding the light caused by diffused reflection within the panel are not satisfactory.
FIG. 4(a) is a schematic cross-sectional view of a TFT substrate with a conventional structure. Upon a transparent insulating film 41 of glass, quartz or the like, a first light-shielding film 42 is formed and thereon, over a first interlayer film 43, a p-Si is formed as an active layer for a TFT. In the active layer, a source-drain is formed by dopant implantation and a channel 45 is formed between the source and the drain. This drawing shows a cross-sectional view taken in the direction of the channel width. Consequently, in this setting, the source and the drain are meant to be above and under the plane of the drawing, respectively. On the active layer, a gate electrode 48 is formed via a gate oxide film 47, and thereon a second light-shielding film 50 which normally also serves as a data line is formed onto a second interlayer film 49. Further, onto those, a third interlayer film 51 is formed and then a pixel electrode, a liquid crystal layer and an opposite substrate, none of which is shown in the drawing, are formed and thereby a liquid crystal panel is accomplished.
When the principal direction of the light emission from a light source is arranged perpendicular to a liquid crystal panel, it is considered that almost all the directions of propagation of the light passing through the liquid crystal panel are normally confined in the directions that make angles not exceeding 30xc2x0 with the direction normal to the surface of the liquid crystal panel. This is disclosed in Paragraph (0065) of Japanese Patent Application Laid-open No. 171101/1996. Therefore, only the light with the direction at an angle up to 30xc2x0 to the direction normal is concerned herein. Upon this, a conventional structure in which the first light-shielding film 42 is formed to have the same width as the channel width has a problem that a reflected light 61 generated by the reflection of an incident light 60 on the reverse of the substrate or a reflected light 62 from the optical system may enter into the channel.
If, in order to overcome this, the width of the first light-shielding film 42 is extended to almost the same width of the second light-shielding film 50, as shown in FIG. 4(b), the reflected lights 61 and 62 are obstructed by the first light-shielding film and stopped reaching the channel. However, in this case, there arises a new problem that a reflected light 63 generated by the reflection of the incident light 60 on the surface of the first light-shielding film may enter into the channel. Further, to eliminate this problem, an increase in the width of the second light-shielding film 50 is required, which results in a reduction of the aperture ratio.
An object of the present invention is to provide a liquid crystal display device capable to prevent a reflected light from the reverse of a substrate and a reflected light from an optical system from entering into a channel therein, without lowering aperture ratio thereof.
Accordingly, the present invention is directed to a liquid crystal display device having, on a transparent insulating substrate, a first light-shielding film, a first interlayer film, a thin film transistor, a second interlayer film and a second light-shielding film, in this order; wherein:
said first light-shielding film is, with a taper-shaped end, in the form of trapezoid, in which the upper side on the side of the thin film transistor is shorter than the lower side on the side of the substrate, and an angle made between the line connecting an end point of said lower side and an end point of a channel in the thin film transistor and the direction normal at said end point of the channel is equal to or more than 50xc2x0; and,
in addition, an angle made between the line connecting an end point of the lower face of said second light-shielding film and a taper starting point of the upper side of said first light-shielding film and the direction normal at said end point of the lower face of the second light-shielding film is equal to or more than 30xc2x0.
In the present invention, by defining the positioning relation among the first light-shielding film formed below the TFT, the second light-shielding film formed above the TFT and the channel and making an end section of the first light-shielding film into the form of a taper, the reflected light from an optical system set on the emission side outside of the liquid crystal panel and the reflected light from the reverse of the substrate on which the TFT is formed are certainly obstructed by the first light-shielding film and besides the light travelled behind the second light-shielding film in an oblique way is reflected by a taper face of the first light-shielding film, and thereby the leakage of the light in the channel of the TFT can be suppressed, along with achieving a higher aperture ratio.